Dimming apparatus for LEDs
A dimming way without turning on/off is to change output voltage for LED or OLED lamp corresponding to dimming percentage without turn on and off LED or OLED lamp; The dimming control signal can come from a keyboard, a phase dimmer, a dimmer button or a computer; The dimming control signal can be sent to controller through PWM signal, phase dimmer or I2C; The output load can be LED lamp or OLED lamp.
The present application claims priority to U.S. Provisional Patent Application No. 61/741,615, filed on Jul. 24, 2012, the entire contents of which are incorporated herein by reference.
BACKGROUNDThe following disclosure is related to electrical circuits and signal processing.
Dimming for lamp is often used with switching power supply. Traditional dimming is realized by two ways:
1. Output current through LED lamp is a train of pulsed square waveform with high frequency; the duty cycle of the pulse current is adjusted to dim the LED lamp. This bring flicker at high frequency.
Output lamp brightness is proportional to output power of the DC to DC or AC to DC converter. Output power P=average(Vo*Io), Vo is constant, Io=IfI (IfI is current at full load) during on time Ton, Io=0 during off time Toff. So P=Vo*average(Io); P=Vo*IfI*(Ton/Ts)=Vo*IfI*D. (D is duty cycle and D=Ton/Ts). Output lamp brightness is proportional to duty cycle because Vo and IfI are constant. So if we want to dim lamp at 10% brightness, we only need to set D=10%. Pupil usually opens wide at darkness and shrinks at brightness. Because the frequency is high, pupil cannot follow the change of light. So strong light stimulate retina during Ton with high frequency, long time the stimulation will hurt eyes and eyesight.
2. The output current is like rectified sinusoidal waveform in which partial of the waveform is 0; the conduction angle of input voltage for lamp is adjusted to dim the LED lamp. This way brings flicker at low frequency around 100 Hz or 120 Hz.
In general and in one aspect, the specification describes a dimming way with power supply to dim the lamp without turning on and off lamp. The new dimming way is to adjust output voltage according to a dimming percentage of brightness.
When you want lamp brighter, you can increase output voltage across LED lamp series; when you want lamp darker, you can decrease output voltage across LED lamp series.
By this way, LED dimming is realized without turning on and off LED lamp. The LED lamp is dimmed without flicker.
Shockley Diode Equation:
I=IS(eV
-
- I is the diode current,
- IS is the reverse bias saturation current (or scale current),
- VD is the voltage across the diode,
- VT is the thermal voltage, and
- n is the ideality factor, also known as the quality factor or sometimes emission coefficient. The ideality factor n typically varies from 1 to 2 (though can in some cases be higher)
- Output power P=Vo*Io, at full brightness Vo=Vf, Vf is the voltage across diode at full brightness.
- So at full brightness, output power Pf=Vf*Io=Vf*Is*(exp(Vf/(n*Vt))−1).
- InPf=In(Is*Vf)+Vf/(n*Vt); (1 is too small compared with exp, so can be neglected)
- let x is the percentage of lamp brightness; Vx, Ix are the voltage and current of the LED lamp to dim the lamp at x percentage of full brightness.
- The power is Px=x*Pf=Vx*Ix=Vx*Is*(exp(Vx/(n*Vt))−1), (1 can be neglected) so we get logarithm for the equation,
In(x*Pf)=In(Is*Vx)+Vx/(n*Vt);
then left side of equation is:
In(x*Pf)=In(x)+In(Pf)=Inx+In(Is*Vf)+Vf/(n*Vt);
In(x)+In(Is*Vf)+Vf/(n*Vt)=In(Is*Vx)+Vx/(n*Vt);
In(Vx/Vf)+(Vx−Vf)/(n*Vt)−In(x)=0; (1)
let u=(Vx/Vf−1), |u|<1, In(Vx/Vf)=In(Vx/Vf−1+1)
In(Vx/Vf)=In(1+u)=u−u*u/2+u*u*u/3−u*u*u*u/4+ . . . +((−1)^(n−1))*((x^n)/n; - |u|<1, we can neglect the item with index above 2, So we get In(Vx/Vf)≈u−u*u/2,
- We rearrange equation (1), In(1+(Vx/Vf−1))+Vf(Vx/Vf−1)/(n*Vt)−Inx=0
- The equation (1) becomes u−u*u/2+Vf*u/(n*Vt)−In(x)=0, we arrange the equation,
U*u/2−(Vf/(n*Vt)+1)*u+In(x)=0; (2)
U=Vf/(n*Vt)+1±sqrt{[Vf/(n*Vt)+1]*[Vf/(n*Vt)+1]−2*In(x)}, - For dimming, x<1, Vx<Vf, u<0,
so u=Vf/(n*Vt)+1−sqrt{[Vf/(n*Vt)+1]*[Vf/(n*Vt)+1]−2*In(x)}, (3) - Substitute u=(Vx/Vf−1) to (3), we get
Vx/Vf=2+Vf/(n*Vt)−sqrt{[Vf/(n*Vt)+1]^2−2*In(x)}
Vx=Vf*(2+Vf/(n*Vt)−sqrt{[Vf/(n*Vt)+1]^2−2*In(x)}) (4) - We can name dimming voltage as Vdim
Vdim=Vf*(2+Vf/(n*Vt)−sqrt{[Vf/(n*Vt)+1]^2−2*In(x)}) (5) - For dimming percentage x,(0<x<100%), we can set voltage across the LED lamp as equation (4) to dim the lamp at x percentage brightness of LED lamp.
LED current at full brightness I(100%)=Is*exp((Vdim(100%)/(n*Vt))−1), (6)
LED current at dimming x%, I(x%)=Is*exp((Vdim(x%)/(n*Vt))−1). (7) - If the total number of LED in series is m, the voltage across LED series is Vdim*m, and the voltage across LED series at dimming x% is
Vdim*m(x%)=m*Vf*(2+Vf/(n*Vt)−sqrt{[Vf/(n*Vt)+1]^2−2*In(x)}). (8) - Numerical analysis may get more accurate result than neglecting the item with index above 2.
Input voltage 101 is either DC or AC voltage. DC to DC or AC to DC converter 102 supply a pulsed DC current to LED lamp with duty cycle corresponding to dimming percentage. Controller 103 receives dimming control signal 104 and sends control signal to converter 102 to control duty cycle of pulsed output current. Feedback 105 feed voltage or current back to controller to regulate out current reach predetermined value during LED on time.
Vo is the output voltage across the LED series, lo is the output current through LED. m*Vf is the voltage across the LED series with number of m LED lamps at full brightness; Vdim*m(x%) is the voltage across the LED series with number of m LED lamps at x% dimming brightness; I(100%) is the LED current at full brightness; I(x%) is the LED current at x% diming brightness.
The dimming control signal 504 discussed can come from keyboard, a potentiometer, a GUI, a memory or a computer. The input to controller 503 can be based on PWM or I2C for DC input voltage.
The dimming control signal 604 discussed can come from phase dimmer, keyboard, a potentiometer, a GUI, a memory or a computer. The input to controller 603 can be based on phase angle, PWM or I2C.
But the control signal is not limited to PWM, phase angle or I2C. The control signal in the invention can come from a phone or a remote controller.
I2C has two lines one is SDA that is data line and SCL is a clock line. Data are exchanged between master and slave.
The output voltage is varied either by changing output voltage divider value or reference voltage. But it is not limited to these. The output voltage change can also be set by programmed software or a table in a digital controller memory.
The lamp is LED or OLED.
Claims
1. An apparatus for dimming lighting comprising:
- a DC voltage source that is either generated from an adapter or a battery, the voltage source operable to supply a DC voltage to a variable voltage converter;
- the variable voltage converter is a DC to DC converter operable to supply a voltage to LED lamps with the voltage value determined by the desired dimming percentage of the LED lamps;
- a dimming input controller operable to receive a feedback signal and change a reference voltage of the power supply to set an output voltage of the converter at the value corresponding to the desired dimming percentage of the LED lamps;
- the dimming input controller is a digital signal process controller that receives a dimming control signal;
- a linear regulator filter controls output of the variable voltage converter to the LED lamps.
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Type: Grant
Filed: Jul 23, 2013
Date of Patent: Apr 12, 2016
Patent Publication Number: 20140028202
Inventor: Wei Zhao (Allen, TX)
Primary Examiner: Michael Zarroli
Application Number: 13/987,414
International Classification: H05B 33/08 (20060101);